Basic casings of electronic devices (such as mobile phones, tablet computers, and multimedia players) may include a casing with a flat outer surface and four sidewalls. A surface treatment method for such a casing may be complicated. Under some circumstance for example, a protective film is required to be formed on and fully wrapped around the casing via a surface treatment method to protect the electronic device from being damaged. To further complicate matters, the protective film may also need to be formed as a pattern or may need to allow for decorative patterning. Therefore, there is room for improvement in the art.